The present invention relates to paper-manufacturing machines.
In particular, the present invention relates to the press section of a paper machine. As is well known such a press section receives the web from the wire and dewaters the web as much as possible so that when the web travels from the press section to the drying section, the load on the drying section will be reduced as much as possible.
In particular, the present invention relates to structure for transferring the web away from the press section when it starts to travel therefrom toward the drying section.
As is well known, particular problems are encountered in the press section because of the desirability of moving the web at high speed so as to achieve a high output while at the same time seeking to dewater the web as much as possible and subjecting the web to as small stress as possible so as to avoid undesirable tearing of the web.
In order to solve the above problems it has been proposed to provide a press section where the web sequentially undergoes a plurality of nips where water is removed from the web, utilizing at these nips rolls capable of efficiently dewatering the web by way of suction as well as various forms of openings formed in the surfaces of the rolls. In addition, in order to fully support the web at all times as it travels through the press section, so as to reduce stressing of the web to a minimum, the press section is arranged in such a way that throughout the entire path of travel of the web through the press section this web is in engagement with felts or rolls so that at no time is the web unsupported as it travels the web section.
However, a particular problem is encountered in detaching the web from the press section as it travels away from the latter toward the drying section.
Thus, the present invention relates in particular to this latter problem especially in connection with that type of press section where the web is continuously supported along its entire path through the press section, although the transfer structure of the invention for detaching the web from the press section is of general utility so that it can also be used with press sections, for example, which do not necessarily provide continuous support for the web throughout the press section.
It is commonly known in paper machines to detach the web from the surface of a rotating press roll while transferring the web, during maintenance of the continuity thereof, to the next treatment stage, with the arrangement being such that the velocity of the web at the latter stage is greater than at the press roll. As a result the detaching of the web is carried out by way of a differential linear web velocity providing a traction in the web. This traction of course stresses the web since it tends to stretch the latter, and the web is not always capable of tolerating such traction since the web under these conditions may break, thus undesirably interrupting the continuous production and resulting in production losses.
One of the characteristic features of modern paper machines resides in the fact that they operate at high speeds. The higher the speed of the machine the shorter the time during which the web is subjected to the effect of pressure at the nips between the rolls of the press section. This factor of course detracts from the dewatering capacity of the press section and it becomes necessary, therefore, in order to render the operation of the press more efficient, to use relatively high pressing forces when the machine operates at high speed. However, this increase in pressure necessarily results in providing a greater adherence of the web to the surface of a smooth press roll engaged by the web, and thus detaching of the web from this smooth press roll becomes difficult. As a result of this latter factor, the tendency of the web to suffer from rupture increases and production losses resulting from breakage of the web increase correspondingly. These problems encountered with ruptures in the web occur particularly at the edge regions of the web.
An additional area where problems are particularly encountered is in connection with detaching of the web from a press roll of the press section in order to start the web on its way to the drying section. For such detachment of the web it is possible to use a pick-up felt and a suction roll cooperating therewith. Such structure is indeed known in connection with transfer of the wet web from the wire to the press section of the machine. However, a conventional suction roll consistent in its design and operation with a wire suction roll or a press suction roll and operating to detach the web from the wire is not suited for use in the press section. The reason for this is that the design and principle of operation of such a roll does not permit it to operate satisfactorily under the particular conditions encountered in detaching the web from the press section so that it will then continue to travel on to the drying section. Such conventional suction rolls have within a foraminous shell a suction box the length of which equals that of the shell, this suction box being connected with a suction system. The suction box is provided with sealing foils which are urged against the inner surface of the roll shell. In order to reduce the extent to which wear takes place between the foils and the surface slidably engaging the same, water is conventionally sprayed onto the interior surface of the suction roll. This water forms seal water cooperating with the sealing foils to achieve the required seal between the latter and the inner surface of the shell while at the same time reducing the friction therebetween. The amount of seal water which may be used can be on the order of 50 liters per minute per meter of machine width. Part of this water to provide a seal while reducing friction is flung by centrifugal force through the perforations or holes in the shell of the roll. As a result this water reaches and is absorbed by the pick-up felt, spreading through the latter and into the paper web itself. In addition, it can happen that as a result of defective nozzles on the spray pipe the water is not uniformly distributed across the machine width, or in other words along the axis of the suction roll. This latter factor will result in a detrimental effect on the moisture profile of the web because the moisture will of course be sharply different at different parts of the web, considered transversely of the machine.
When the web is detached from the wire, the water content of the web is so high that introduction of seal water into the pick-up felt and partly into the web itself usually is of no particular consequence. On the other hand, if in the press section there is a detaching suction roll of such conventional design, then the quantity of seal water as compared with the quantity of the water remaining in the paper web at the last press nip is so great that it will substantially impair the operational effeciency of the press. For example, if the dry matter content of the web increases at the last press nip from 37% to 40%, with a machine speed of 800 m/min and a basis weight of 50 g/m.sup.2 of the web, then it follows that water is withdrawn from the web at the press section at a rate of about 8 liters per minute per meter of machine width. However, an equal amount of water or even more water, may be reintroduced into the web from the seal water of the detaching suction roll situated subsequent to the last press nip. Such a detaching suction roll placed in the press section cannot be given a relatively large diameter. The necessarily small diameter of this roll results in a considerable centrifugal force, and therefore it cannot be avoided that water will be flung about through the perforations of the shell into the paper web in great abundance.
As has been pointed out above, a large number of breaks in the web will start at the margins of the web during detachment thereof from the press section. This large number of breaks which start at the margin results from the fact that the web margins may lack homogeneity and may even be inferior in strength as a result of unsatisfactory operation of the headbox and of the wire section. Moreover, the ease with which the web starts to tear at its margins is at least in part a result of the air current created by the rotating rolls and the like which cause fluttering of the web.